The present invention relates to a method and apparatus for delivering precursors for use in chemical vapor deposition (CVD) or atomic layer deposition (ALD) processes, and more particularly, to the use of an energy source to vaporize and deliver the precursors to a process or reaction chamber without subjecting the precursors to bulk thermal decomposition.
Chemical vapor deposition (CVD) has been extensively used for preparation of films and coatings in semiconductor wafer processing. CVD is a favored deposition process in many respects because of its ability to provide highly conformal and high quality films at relatively fast processing times. Further, CVD is beneficial in coating substrates of irregular shapes, including the provision of highly conformal films even with respect to deep contacts and other openings.
Atomic layer deposition (ALD) is a relatively new process which is becoming favored as a method for achieving uniform thin deposition layers. While ALD is a slower process than CVD, ALD allows the use of precursors which are higher in reactivity because the chemical species are injected independently into an ALD reactor, which in turn allows processing at lower temperatures than conventional CVD processes.
Standard CVD and ALD processes employ precursor sources in vaporization chambers that are separated from the process or reactor chamber where the deposition surface or wafer is located. Liquid precursors are typically placed in bubblers and heated to a temperature at which they vaporize, and the vaporized liquid precursor material is then transported by a carrier gas passing over the bubbler or through the liquid precursor. The vapors are swept through a gas line to the process or reaction chamber for depositing a CVD or ALD film on a heated substrate or wafer. Many techniques have been developed to precisely control this process, and the amount of material transported to the process chamber can be precisely controlled by, for example, the temperature of the liquid precursor reservoir and by the flow of the carrier gas bubbled through or passed over the reservoir.
For example, Mikoshiba et al, U.S. Pat. No. 5,476,547 describes a gas feeding device which bubbles a carrier gas through a liquid organometallic precursor. Huston et al, U.S. Pat. No. 6,179,277 and Vaartstra et al, U.S. Pat. No. 6,244,575, both describe two-step vaporization systems for liquid organometallic precursors.
However, similar techniques have not been adequate for vaporizing solid precursors suitable for depositing CVD and ALD films. For illustration, similar techniques may include bulk sublimation of the solid precursor with transport of the vaporized solid precursor to the process chamber using a carrier gas in a manner similar to the transport of the vaporized liquid precursor. Solid precursors have generally been considered to be poor choices for CVD and ALD processes due to the difficulty of vaporizing, i.e. subliming, a solid at a controlled rate to provide a reproducible flow of vapor. However, there are many off-the-shelf solid precursors available, particularly solid organometallic precursors, which, if they could be delivered effectively and reproducibly, could be used in CVD and ALD processes.
Lack of control of solid precursor sublimation is due, at least in part, to the changing surface area of the bulk solid precursor as it is vaporized. Such a changing surface area when the bulk solid precursor is exposed to sublimation temperatures produces a continuously changing rate of vaporization, particularly for thermally sensitive compounds. This ever changing rate of vaporization results in a continuously changing and nonreproducible flow of vaporized solid precursor delivered for deposition to the process chamber. As a result, film growth rate and composition of such films deposited on wafers in the process chamber using such vaporized solid precursors cannot be controlled adequately and effectively.
Therefore, it is important to precisely control the exposure of the solid precursors to elevated temperatures to avoid bulk decomposition of the solid precursor material. However, many solid precursors, such as organometallic precursors, decompose slowly when held near their sublimation temperatures. This prevents the use of a continuously heated chemical ampoule or other vessel to maintain an elevated vapor pressure.
Accordingly, there remains a need in the art for a vapor delivery system for delivering both solid and liquid precursors, particularly thermally sensitive precursors for use in a CVD or ALD process, at a precisely controllable rate and without bulk decomposition of the precursor material during vaporization.
The present invention meets that need by providing a method and apparatus for delivering gaseous precursors to a CVD or ALD process that overcomes the above-mentioned problems by controlling the rate of vaporization at the surface of the precursor while avoiding bulk thermal decomposition of the precursor. Thus, the precursor is a phase change material which undergoes a change in phase from solid or liquid to a gaseous vapor during processing.
According to one aspect of the present invention, a method is provided for vaporizing a material such as a precursor in which a precursor vaporizer, preferably in the form of an energy source, is used to vaporize a portion of a precursor. The precursor is vaporized by exposing the surface of the precursor to the energy source. By xe2x80x9cenergy sourcexe2x80x9d, it is meant a source which is capable of increasing temperature to provide evaporation or sublimation of a material such as a precursor. Preferably, the energy source is selected from the group consisting of a gas, a radio frequency coupling device, and an infrared irradiation source.
Where the energy source comprises a gas, the gas preferably has a temperature of at least about 20xc2x0 C. higher than the precursor. Generally, the temperature of the gas will be between about 10xc2x0 C. to about 300xc2x0 C., and more preferably, between about 50xc2x0 C. to about 300xc2x0 C. The gas is preferably a carrier gas which is non-reactive with the precursor. Suitable carrier gases include those selected from the group consisting of nitrogen, helium, and argon, or a combination thereof.
The precursor is preferably present in solid or liquid form and undergoes a phase change to a gaseous vapor when exposed to the energy source. The energy source vaporizes the surface of the precursor without heating the entire volume of the precursor such that substantially no thermal decomposition of the remaining precursor occurs. By xe2x80x9csubstantially no thermal decompositionxe2x80x9d it is meant that the majority of the mass of the precursor maintains its thermal stability. In a preferred embodiment, the vaporized portion of the precursor is then transported to a deposition chamber such as a chemical vapor deposition or atomic layer deposition chamber for further processing.
Precursors suitable for use in the method of the present invention include both organic and inorganic metal-containing compounds. The precursors may be either in a solid or liquid form, depending upon the temperature at which the precursors are maintained and undergo a phase change during processing. As used herein, the term xe2x80x9cmetal organicxe2x80x9d includes metal organic compounds having a central atom bonded to at least one carbon atom of a ligand as well as compounds having a central atom bonded directly to atoms other than carbon in a ligand. Preferred precursors include metal organic precursors which have at least one metal selected from the group consisting of Sr, Ba, Sc, Y, La, Ce, Ti, Zr, Hf, Pr, V, Nb, Ta, Nd, Cr, W, Pm Mn, Re, Sm, Fe, Ru, Eu, Co, Rh, Ir, Gd, Ni, Tb, Cu, Dy, Ho, Al, Tl, Er, Sn, Pb, Tm, Bi, Yb, and Si.
For example, where it is desired to deposit a titanium-containing material, the precursor will contain titanium (Ti). Suitable precursor compounds containing titanium include tetrakis-dimethyl aminotitanium, tetrakis-diethyl aminotitanium, bis(2,4-dimethyl-1,3-pentadienyl) titanium cyclopentadienyl cycloheptatrienyl titanium, dicyclooctatetraene titanium, and biscyclopentadienyltitanium diazide. Known titanium-containing liquid precursors include titanium tetrachloride, and tetrakisdimethylamidotitanium (TDMAT). Known silicon-containing precursors include tetraethoxysilane, tetraethyl orthosilicate (TEOS). Where it is desired to deposit different metals, other known precursor compounds may be utilized.
In embodiments where the method is used to vaporize a solid precursor, the method includes exposing the surface of the solid precursor to an energy source such that the solid precursor is sublimated at its surface, preferably without heating the entire volume of the precursor.
An apparatus used in accordance with the present invention for delivering gaseous precursors includes a temperature-controlled vessel containing a solid or liquid precursor and an energy source. The energy source is preferably selected from the group consisting of a heated gas, a radio frequency coupling device, and an infrared radiation source. Preferably, the precursor is contained in a temperature-controlled vessel such as an ampoule. The vessel also includes an outlet configured to pass vaporized precursor therethrough. The apparatus further includes a temperature-controlled conduit communicating with the outlet configured to deliver the vaporized precursor to a deposition chamber such as a chemical vapor deposition or atomic layer deposition chamber.
Where the energy source comprises a gas, the vessel preferably includes an inlet for receiving the gas. The temperature of the gas entering the vessel is preferably at least about 20xc2x0 C. higher than the temperature of the bulk precursor compound contained in the vessel. While the gas need not be heated if that condition is met, typically the apparatus also preferably includes a heater connected to the inlet for heating the gas prior to entering the vessel. The gas is preferably heated to a temperature of between about 10xc2x0 C. to about 300xc2x0 C. prior to entering the vessel.
Where the energy source comprises a radio frequency coupling device or an infrared irradiation source, these sources are preferably located outside the vessel and act on a surface of the precursor in the vessel.
Accordingly, it is a feature of the present invention to provide a method and apparatus for vaporizing and delivering precursors to a chemical vapor deposition or atomic layer deposition chamber with substantially no bulk thermal decomposition of the precursor. These, and other features and advantages of the present invention, will become apparent from the following detailed description, the accompanying drawings, and the appended claims.